The objective of this proposal is to develop a novel 3-D ultrasound system for in vivo imaging of targets near the skin surface such as lesions in the breast. Typical ultrasound systems use a one-dimensional linear array operating in the 7-13 MHz range to produce two-dimensional tomographic scans of the breast. The results of such a scan can depend heavily on skill of the user since he/she must mentally integrate several images to gain an understanding of the three-dimensional anatomy. In addition, the success of ultrasound-guided biopsies can largely depend on the skill of the operator since the needle must be perfectly aligned within a single scan plane. However, a system capable of in vivo imaging of the breast in three dimensions could help alleviate some of these problems and therefore show improved clinical value over today's commercial systems. Using an innovative combination of synthetic aperture methods and a novel interconnect scheme, a 256 x 256 10 MHz 2-D array will be designed, prototyped and fabricated. This new system will allow for the synthesis of a fully sampled 2-D array capable of dynamic focusing in azimuth and elevation. To demonstrate proof of concept, a prototype transducer will be integrated with a 64 channel ultrasound research platform used to gather the necessary echo information needed to produce rectilinear 3-D volumes. A series of water tank and tissue phantom experiments will be performed to evaluate imaging performance in terms of resolution, contrast, and signal-to-noise ratio.